This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The proposed project will longitudinally address two important questions: (1) How do maternal condition and life history tradeoffs affect lactational investment and subsequent reproduction, (2) How do differences in lactational investment program infant developmental trajectories? Studies of maternal investment strategies in primates have focused mainly on the behavior of lactating females and their infants, documenting the frequency of suckling, proximity maintenance, and the dynamics of weaning. To date we know little about factors that influence the primary form of maternal investment, milk synthesis. Previous research in rhesus macaques has demonstrated that milk synthesis is influenced by maternal life history, reproductive experience, and infant sex. Additionally this research suggests that the milk energy available soon after birth is a nutritional cue that calibrates the infant's behavior and temperament to environmental and maternal conditions, and that this calibration has consequences for reproductive fitness. The proposed study will augment and extend these findings in the large captive population of rhesus macaques at the California National Primate Research Center. Longitudinal comparisons of lactation performance within and among mothers and the consequences for infant growth and behavioral development will indicate how mothers manipulate lactational investment in relation to their own, and their infant's, physiological and social condition. This research project will provide the first systematic longitudinal information investigating all dimensions of mammalian lactation strategies within a single species (milk synthesis, nursing behavior, duration of lactation until weaning, and number and sex-ratio of infants supported during lactation). As such this study will explore fundamental aspects of lactation that have previously not been investigated, and advance our understanding of how selection has shaped maternal investment strategies. The data from this study will directly address critical issues affecting the health of mothers and their infants. Breastfeeding rates in the US and throughout the world are significantly below optimum, contributing to long-term deleterious developmental and health consequences for infants. Advances in understanding how constraints during lactation influence the nutritional value of the mother's milk, under controlled research conditions will provide insight into similar challenges for human mothers and provide vital information for current global health concerns. This research will provide valuable resources, in the form of archived samples and public databases, for other researchers studying evolutionary theory (sex-biased investment strategies) to mechanistic explanations (the role of milk cortisol influencing infant metabolic programming).